1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on a sheet of paper being transported through a sheet transport path such as electrophotographic copying machines and printers, and more particularly to an image forming apparatus which performs a plurality of image forming operations on one sheet of paper.
2. Description of Related Art
As image forming apparatus such as electrophotographic copying machines and printers, it is well known that the apparatus is provided with a both-side mode for forming images on both sides of a copy sheet and a composite mode for forming images a plurality of times on one side of a copy sheet.
In such an apparatus, there is one which is provided, for instance, with a paper re-feeding transport path where a sheet of paper discharged from an image forming path in which the paper is transported for forming images thereon is returned to the image forming path again. In this paper re-feeding transport path, a paper is returned as it is when composite mode is carried out, and in case of both-side mode, a paper is switched back to return by inverting the paper.
However, when an image fOrming operation is successively carried out for forming images on both sides of a plurality of sheets of paper, for instance, if both-side image forming operation is performed one sheet by one sheet, the image forming operation is suspended while a sheet is being transported in the paper re-feeding transport path. It, therefore, causes to considerably lower throughput, i.e. the number of sheets per unit hour which can be processed in the image forming operation.
U.S. Pat. No. 4,499,500 and U.S. Pat. No. 4,568,169 disclose the image forming apparatus capable of solving such defects. The apparatus disclosed in the U.S. Pat. No. 4,499,500 is arranged to feed another sheet of paper for image forming process when a sheet is being transported in the paper re-feeding transport path, while the apparatus disclosed in the U.S. Pat. No. 4,568,169 is arranged to store a plurality of sheets of paper both in the image forming path and the paper re-feeding transport path with further arrangement of varying transport spaced in the paper re-feeding transport path corresponding to the size and the number of sheets stored.
Generally, however, the maximum number of sheets which can be stored in the paper transport path varies according to the size of paper to be transported, interval between papers and the length of paper transport path selected, etc. Accordingly, in the apparatus disclosed in the U.S. Pat. No. 4,499,500, the number of sheets of paper to be stored can only be set based on the least condition. It is, therefore, impossible to carry out effective image forming operations corresponding to every possible image forming condition.
The apparatus disclosed in the U.S. Pat. No. 4,568,169 is capable of optionally setting the number of sheets of paper to be stored by selecting a transport speed in the paper re-feeding path. For instance, however, it is difficult for the operator to judge the maximum storable number of sheets of paper taking into account the various conditions as above-mentioned. Accordingly, in setting a storable number of sheets of paper, it is preferable to select the number of sheets within a range of storable number of sheets under the least condition for preventing troubles. In this regard, the apparatus disclosed in the U.S. Pat. No. 4,568,169 has the same problem as that of the U.S. Pat. No. 4,499,500.